Short-Term Borrowings - Mortgage Loan Participation and Sale Agreement (Details)	12 Months Ended
	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jul. 30, 2021 USD ($) item	Nov. 01, 2018 USD ($)
During the period:
Total interest expense	$ 4,153,000	$ 4,933,000	$ 8,874,000
Carrying value:
Mortgage loan participation and sale agreement secured by mortgage loan participation certificates	479,845,000	521,477,000
Fair value of loans pledged to secure	505,716,000	545,500,000
Mortgage Loan Participation and Sale Agreement member
During the period:
Average balance	$ 249,255,000	$ 226,689,000	$ 244,203,000
Weighted-average interest rate (as a percent)	1.39%	1.88%	3.42%
Total interest expense	$ 4,153,000	$ 4,933,000	$ 8,874,000
Maximum daily amount outstanding	532,819,000	540,977,000	548,038,000
Carrying value:
Unpaid principal balance of mortgage loan participation and sale agreement secured by mortgage loan participation certificates	479,845,000	521,477,000
Mortgage loan participation and sale agreement secured by mortgage loan participation certificates	$ 479,845,000	$ 521,477,000
Weighted average interest rate (as a percent)	1.48%	1.39%
Fair value of loans pledged to secure	$ 505,716,000	$ 545,500,000
Amortization of debt issuance costs	688,000	662,000	514,000
Note Payable
Carrying value:
Unamortized debt issuance costs	(2,378,000)	(4,160,000)
Amortization of debt issuance costs	2,200,000	1,800,000	$ 1,800,000
Syndicated Repurchase Agreement
Short-term Debt [Line Items]
Number of indirect wholly owned subsidiaries entered into agreements				2
The number of existing variable funding note repurchase agreements | item				2
Previous maximum aggregate purchase price				$ 400,000,000
Maximum aggregate purchase price				500,000,000
Syndicated Repurchase Agreement SAR
Short-term Debt [Line Items]
Maximum aggregate purchase price				$ 600,000,000
Revolving credit agreement | Note Payable
Carrying value:
Maximum loan amount					$ 150,000,000
Increase in loan amount	$ 0	$ 0